‘Never abashed’: Shia LaBeouf and Rosie Huntington-Whiteley in Transformers: Dark of the Moon.

There's a celebrated clip from the American cartoon South Park in which a general turns to Hollywood for ideas on how to defeat terrorists. Director Michael Bay says he has some ideas. "An 18-wheeler spins out of control and it's like braaah-sssh," he offers, shaking with excitement. "Then these motorcycles burst into flames and jump over these helicopters and it's all braaah-sssh..."

The general looks at the cartoon Bay and says: "Those aren't ideas, those are special effects." Bay, abashed, replies: "I don't understand the difference."

In real life, however, Michael Bay is never abashed. The third film in the Transformers series, Dark of the Moon, is loud (it's in Dolby 7.1, which is a hell of a lot of Dolby) and undeniably spectacular in its 3D imagery. I'd even go so far as to say that some of it is entertaining and thrilling. But are there any ideas here? Other than to make more than $1.5bn, what is the point of a Michael Bay movie?

Although his tyrannical on-set style could be likened to an old-fashioned Hollywood director such as Cecil B DeMille, Bay basically likes to blow stuff up. In Dark of the Moon, downtown Chicago is ripped to shreds by the war between the Autobots (good) and the Decepticons (nasty), with humans and our puny buildings caught in the crossfire.

Having watched three Transformers films, I think I'm getting the hang of what's going on – I even know the names of at least two Autobots: Optimus Prime and Bumblebee. These giant robot machines, which move among us disguised as cars and trucks but can transform back at any second, have found a new home on Earth after the destruction of their planet, Cybertron. Housed in a government bunker, they're training earthlings to deal with an inevitable Decepticon invasion and (to be said in a booming Barratt Homes voice) "prevent mankind from bringing harm to itself".

Sam Witwicky, played by the unappealing Shia LaBeouf, is the human the Autobots have chosen to be their chief confidant, and by now he's graduated from college and got rid of his hot, biker-chick girlfriend, who used to be played by Megan Fox, dropped from the franchise for hating Michael Bay and for likening him to Hitler (rather than, say, DeMille).

Fox has been replaced by a British underwear model named Rosie Huntington-Whiteley, with whom Bay worked creating adverts for lingerie chain Victoria's Secret in which she purred on a pool table, sprouted angel wings and draped herself on car bonnets, all while wearing knickers and bra. Not much more is required of her here, except that now: Rosie talks.

Or, rather, she gabbles, in that English posh girl, Liz Hurley way that sounds perfectly fine on an afternoon at Henley but which renders her practically unintelligible when surrounded by American accents and falling masonry. In any case, we're first introduced to Rosie via a close-up of her bum, segueing straight from the film's opening sequence and titles on to the pert buttocks and underwear of our heroine. For Michael Bay, exposition must always give way to exhibitionism.

To be fair, the film's opening moments are neatly done in the bombastic Bay style. They posit that the Apollo moon landings were in fact top secret missions to recover alien craft that US radars had spotted landing on the lunar surface several years earlier. (It reminds me of a long-mooted comic action film called Iron Sky, in which fleeing Nazis landed on the moon in 1945 and have been waiting 70 years to launch their revenge, but I don't know what's happened to that one.) We get men walking into rooms and barking: "Get me the president", and faded reconstructions of Kennedy and Nixon in the Oval Office, followed by a well-constructed moon walk in which our astronauts discover a crashed Autobot spacecraft.

It is from this moon wreckage that the Autobot leader, Sentinel Prime, returns, using the voice of Leonard Nimoy, and Bay somehow contrives a cameo out of the real Buzz Aldrin now meeting Prime, a "fellow space explorer". Thereafter, the plot is of little consequence or coherence, although much improved from the second film, which blew up the Pyramids. At some point, Sam Witwicky gets a job with an electronics firm run by billionaire John Malkovich, who gets all enraged by one of his staff drinking from a red coffee cup. "It is a visual and therefore a visceral betrayal," he yells. Malkovich doesn't hang around very long, handing over hammy duties to John Turturro.

Frances McDormand soon bursts in as some sort of defence secretary. Playing it with the sang-froid of Fargo's Marge Gunderson, she handles all this nonsense remarkably well.

If Bay has at last solved the Transformers' sense of scale (this time, you can actually see the entirety of the robots during their metal smackdowns), he has no sense of timing and lets the final showdown go on for what feels like an hour.

Usually in his climaxes, Bay blows up Paris too, probably for having the temerity to be a place where film-making is considered an art form, but the French aren't included in this film's final orgy of destruction porn. Instead, we get Rosie with her tight jeans and her hornet-stung lips, framed against crumpled skyscrapers and rubble, her hair flying like the American flag at Iwo Jima. Bay Theory is immutable, powerful and undeniable, a simple summer blockbuster equation: phwoar + braaah-sssh = kerching!